A number of attempts for producing high modulus fibers have been made. See Ciferri, A., and Ward, I. M., Eds., Ultra-High Modulus Polymers (London: Applied Science, 1979). Zachariades, A. E., and Kanamoto, T., J. Appl. Polym. Sci., 35, 1265-1281 (1988); Smith, P., et al., Polym. Bull., 1, 733-736, (1979); Pennings, A. J., et al., Pure Appl. Chem., 55, 777-798 (1983); Smith, P., and Lemstra P. J., J. Mater. Sci., 15, 505-514 (1980); Leenslag, J. W., and Pennings, A. J., Polymer, 28, 1695-1702 (1987); Gogolewski, S., and Pennings, A. J., Polymer, 26, 1394-1400 (1985). Two general methods exist for producing high modulus fibers. The first method involves the synthesis and assembly of polymers with inherently rigid and linear backbone molecular structures. The second method transforms conventional inherently flexible semicrystalline polymers that usually have relatively low modulus and strength, into highly oriented materials. In the first case, para-substituted aromatic rings in the polymer backbone are used. These rigid polymers can produce fibers of very high stiffness and strength, either by wet spinning to produce aramid fibers or by melt spinning if thermotropic liquid crystalline polymers are used. In the second case, flexible chains of semicrystalline polymers must be converted into highly oriented and extended chain conformations. The main technique for developing an oriented and extended polymer structure is the drawing process. Ultra drawn high molecular weight polyethylene (UHMWPE) is one good example. By drawing UHMWPE more than 200 times, moduli approaching the theoretical value for the draw ratio have been obtained. See Zachariades, A. E., and Kanamoto, T., J. Appl. Polym. Sci., 35, 1265-1281 (1988).
Unlike polyethylene, polyamides have hydrogen bonds between molecular chains. Hydrogen bonds play an important role to make polyamides engineering plastics. At the same time, hydrogen bonds prohibit high draw ratio processing in polyamides, which have a maximum draw ratio of approximately 5. See Postema, A. R., and Smith, P., Polym. Commun., 31, 444-447 (1990). Therefore, highly oriented polyamides obtained by tensile drawing must be made either by suppression of crystallinity or by modification of the number and strength of the hydrogen bonds between their chains.
Many researchers have attempted to overcome the low maximum draw ratio of polyamides by using various processing techniques, such as, plasticizers, dry spinning, gel spinning, wet spinning, and zone drawing and annealing. See Chuah, H. H., and Porter. R. S., Polymer, 27, 241-246 (1986); Kanamoto, T., et al., J. Polym. Sci. Polym. Phys. Edn., 20, 1485-1496 (1982); Chuah, H. H., and Porter, R. S., Polymer, 27, 1022-1029 (1986); Cho, J. W., et al., J. Appl. Polym. Sci., 62, 771-778 (1996); Kunugi, T., et al., Polymer, 23, 1193-1198 (1982); Kunugi, T., et al., Polymer, 23, 1199-1203 (1982); Kunugi, T., et al., Polymer, 24, 1983-1987 (1983); Kunugi, T., et al., J. Appl. Polym. Sci., 67, 1993-2000 (1998); Suzuki, A., et al., Polymer, 39, 1351-1355 (1998); Suzuki, A., et al., Polymer, 38, 3085-3089 (1997); Suzuki, A. and Ishihara, M., J. Appl. Polym. Sci., 83, 1711-1716 (2002); Smook, J., et al., J. Appl. Polym. Sci., 41, 105-116 (1990).
Aliphatic polyamides like nylon 6 and nylon 6,6 are important commercial thermoplastics with good mechanical properties. In applications needing increased performance, however, such as, for example, in ballistic vests, nylon tire cords, ropes, safety nets, and parachutes, fibers with even more superior qualities are desired. Thus, there is a continued need for improved methods to produce high modulus nylon fibers.